A piezoelectric material generates an electric charge in response to an applied mechanical stress. This so-called piezoelectric effect may be utilized to generate a voltage across the surfaces where the mechanical forces are applied to the piezoelectric material. The piezoelectric effect is reversible such that a voltage applied across piezoelectric material results in a dimensional change of the piezoelectric material. By way of the piezoelectric effect, piezoelectric materials convert mechanical energy into electrical energy and vice versa. Importantly, piezoelectric materials with millimeter or even micron-sized dimensions may perform useful energy conversion. Furthermore, piezoelectric materials are relatively inexpensive. Piezoelectric materials are used in, for example, motors, energy harvesting devices, and sensors.
In a piezoelectric motor, a voltage bias is applied across a piezoelectrical material to change a dimensional property of the piezoelectric material. For example, application of an oscillating voltage bias produces a dimensional oscillation such as a repeating expansion and contraction of the piezoelectric material.
In the emerging technology of piezoelectric-based energy harvesting, a piezoelectric material is placed in an ambient environment that applies pressure to the piezoelectric material, resulting in production of electricity. In one example, a piezoelectric-based energy-harvesting device is embedded in a walkway to harvest energy from pressure applied to the walkway by pedestrians, or mounted to a person to harvest energy from dimensional changes of the piezoelectric material caused by the person moving. In another example, the rotation of a wind turbine with a diameter of only about ten centimeters flexes a piezoelectric material to produce electricity. The efficiency with which a piezoelectric material converts mechanical energy into electrical energy affects the output power of an energy-harvesting device and is therefore a critical parameter.
A piezoelectric pressure/stress sensor produces a voltage across the piezoelectric material in response to pressure or stress exerted on the piezoelectric material. The sensitivity of such a sensor is determined by the magnitude of pressure/stress required to produce a measurable voltage.